Electronic devices, such as semiconductor devices, and photonic devices, such as light emitting and solar devices, are often formed by, among other things, depositing thin films of materials onto a substrate. Techniques for depositing films include atomic layer deposition (ALD). ALD may be desirable for various applications, because ALD processes are self-limiting, allowing precise, conformal deposition of material onto a substrate.
Atomic layer deposition typically includes gas-phase deposition of a film using two or more precursors, wherein one or more of the precursors may be activated, using, for example, a direct or remote plasma. A typical ALD process includes deposition using two precursors. In this case, a sequence often includes exposing a surface of a substrate to a first precursor to react with species on the surface of the substrate in a reaction chamber, purging the reaction chamber to remove the first precursor, exposing the surface of the substrate to a second precursor to react with species from the first precursor in the reaction chamber, and removing the second precursor from the reaction chamber. If each precursor is not sufficiently purged prior to the introduction of the other precursor into the reaction chamber, undesired gas-phase reactions may occur, which may, in turn, cause undesired particle formation or non-uniformity of the deposited film. Additionally or alternatively, insufficient purging of a precursor can result in depletion of a subsequently-introduced precursor.
During an ALD deposition process, one or more of the precursors may adsorb onto or be absorbed into a film formed on a surface of the reaction chamber and subsequently outgas during a cycle in which the other precursor is introduced into the reaction chamber. In this case, undesired gas-phase reactions may occur as one precursor outgasses as the other flows through the reaction chamber.
By way of example, when ALD is used to form a metal oxide film on a surface of a substrate, a method to form the metal oxide film may include exposing the substrate surface to a metal-containing precursor to react with chemical species on the substrate surface, purging the reaction chamber, exposing the substrate surface to an oxidant, such as oxygen, activated oxygen, ozone, water, or alcohol, and then purging the reaction chamber. These steps are repeated until a desired amount of the metal oxide is formed on the surface of the substrate. During these steps, the metal oxide film material is also deposited onto surfaces on the reaction chamber—in addition to the substrate surface. If the oxidant is absorbed into or adsorbed onto the material deposited onto the surface of a reaction chamber, the oxidant may not be completely purged from the reaction chamber during a purge step, and may therefore outgas during a subsequent step of exposing the substrate surface to a metal-containing precursor. As the oxidant outgasses, it may react with the metal-containing precursor during the step of exposing the substrate surface to a metal-containing precursor, resulting in undesired non-uniformity of the film across the substrate surface and/or particle formation within the reaction chamber.
To mitigate the outgassing of a precursor, the ALD process may be run at a relatively low pressure—e.g., less than 0.01 Torr—to enhance purging of the precursors. However, reducing the operating pressure of the reaction chamber to such low pressures generally decreases the throughput of the ALD process, and thus systems and methods operating at these pressures are not well suited for production of devices. Accordingly, improved methods and systems for mitigating outgassing from a reaction chamber during gas-phase reactions are desired.